It is generally known that cigarette smoke contains many harmful substances, among them carbon monoxide. Therefore, there is a great interest in the industry to produce cigarettes the smoke from which contains considerably fewer harmful substances. To reduce the amount of such substances, cigarettes are often equipped with filters, typically formed from cellulose acetate. These filters, however, are not suitable for reducing the carbon monoxide content in the smoke from the cigarette, as cellulose acetate cannot absorb carbon monoxide. Various proposals for incorporating catalysts into the filter have so far not been successful, partially for functional and partially for economic reasons.
It is also known to dilute the smoke generated in the cigarette with an air flow flowing through a perforation in the tipping paper. In this regard, the carbon monoxide content of the cigarette smoke can indeed be reduced, however at the price of diluting the substances which determine the taste of the cigarette and thus of having a negative influence on the taste sensation of the cigarette and consumer acceptance.
The substances in cigarette smoke are determined by a method in which the cigarettes are smoked according to standardized protocols. Such a method is, for example, described in ISO 4387. In this regard, the cigarette is at first lit at the start of the first puff, and then each minute a puff is taken at the mouth end of the cigarette with a duration of 2 seconds and a volume of 35 cm3, following a sinusoidal puff profile. The puffs are repeated until the length of the cigarette drops below a length defined in the standard. The smoke flowing from the mouth end of the cigarette during the puffs is collected in a Cambridge Filter Pad and this filter is afterwards chemically analyzed for its content of various substances, for example nicotine. The gas phase flowing out of the mouth end of the cigarette and through the Cambridge Filter Pad during the puffs is collected and also chemically analyzed, for example, to determine the carbon monoxide content in the cigarette smoke.
During standardized smoking, the cigarette is thus in two different states of flow. During the puff, there is a considerable pressure difference, typically in the range from 200 Pa to 1000 Pa, between the inner side facing the tobacco and the outer side of the cigarette paper. This pressure difference causes air to flow through the cigarette paper into the tobacco part of the cigarette and dilutes the smoke generated during the puff. During this phase, which last for 2 seconds per puff, the extent of dilution of the cigarette smoke is determined by the air permeability of the paper. The air permeability is determined in accordance with ISO 2965 and indicates which air volume per unit time, per unit area and per pressure difference flows through the cigarette paper, and thus has the dimension cm2/(min cm2 kPa). It is also referred to as the CORESTA Unit (CU, 1 CU=1 cm3/(min cm2 kPa)). With this value, the rod ventilation of a cigarette is controlled, that is, the air flow which flows through the cigarette paper into the cigarette during a puff on the cigarette. Typically, the air permeability of cigarette paper is in the range from 0 CU to 200 CU, wherein the range from 20 CU to 120 CU is generally preferred
In the time period between the puffs, the cigarette smolders without any appreciable pressure difference between the inside of the tobacco part of the cigarette and the surroundings, so that the gas transport is determined by the difference in gas concentration between the tobacco part and the surroundings. In this regard, carbon monoxide can also diffuse out of the tobacco part through the cigarette paper and into the ambient air. In this phase, which lasts for 58 seconds according to the method described in ISO 4387, the diffusion capacity of the cigarette paper is the relevant parameter for the reduction of carbon monoxide.
The diffusion capacity is a transfer coefficient and describes the permeability of the cigarette paper for a gas flow which is driven by a concentration difference. More precisely, it designates the diffusion capacity of the volume of gas through the paper per unit time, per unit area and per concentration difference and thus has the unit cm3/(s.cm2)=cm/s The diffusion capacity of a cigarette paper for CO2 can, for example, be determined with the CO2 Diffusivity Meter from the company Sodim and is closely related to the diffusion capacity of cigarette paper for CO.
It can be seen from the above considerations that the diffusion capacity of the cigarette has an independent, important significance for the carbon monoxide content in cigarette smoke and that the levels of carbon monoxide in cigarette smoke can be reduced by increasing the diffusion capacity. This is of particular relevance with respect to the self-extinguishing cigarettes known in the prior art, for which comparably high values of carbon monoxide are observed. In such cigarettes, burn-retardant stripes are applied to the cigarette paper, to achieve self-extinguishment in a standardized test (ISO 12863). This or a similar test is, for example, a part of legal requirements in the USA, Canada, Australia and the European Union. The increased values of carbon monoxide are caused by the fact that carbon monoxide can diffuse through the burn-retardant stripes out of the cigarette only to a very small extent. Thus, it would be of great advantage to provide a cigarette paper which could compensate for this undesired side-effect.
In practice, however, it turns out to be extremely difficult to adjust the diffusion capacity independently of the air permeability of the paper in the paper production process. The air permeability itself, however, is in most cases the subject of paper specifications which the cigarette manufacturers have to comply with, so that—under this requirement—the diffusion capacity results practically from the paper production process and can only be varied within a very small range (compare also B.E.: The influence of the pore size distribution of cigarette paper on its diffusion constant and air permeability, SSPT17, 2005, CORESTA meeting, Stratford-upon-Avon, UK). The air permeability as well as the diffusion capacity are determined by the porous structure of the cigarette paper, and so there exists a relationship between these parameters which is approximately given by D*˜Z(1/2), wherein D* is the diffusion capacity and Z the air permeability. This relationship holds above all in good approximation if the air permeability of the paper is primarily adjusted by refining the pulp fibers.
From the prior art, various approaches are known for increasing the diffusion constant of the cigarette paper by adding thermally instable substances (WO 2012013334) or by selecting the mean size of the filler material particles (EP 1450632, EP 1809128). Despite such tests, there is still a lack of a possibility to substantially increase the diffusion capacity for a given air permeability.